Symptomatic adult and pediatric structural heart disease (such as valvular heart disease or cardiac septal defects) affects more than 2.9% of the US population, not including cardiomyopathies and rhythm disorders. Because of procedural morbidity, only a minority are selected for surgical relief of symptoms. Nonsurgical repair of structural heart disease is possible using image guidance and newer devices such as transcatheter aortic valves, mitral valve repairs, and intracardiac occluders. Most are guided by X-ray fluoroscopy and adjunctive 2D intracardiac or 3D transesophageal echocardiography (“TEE”). While available transesophageal and intracardiac echo systems are suitable to assess target pathology immediately before and after treatment, they are unsuitable to guide catheter manipulations during therapeutic procedures. Catheters and target pathology constantly move outside the 2D slices and limited 3D volumes depicted by current echo systems, which also are constrained by interposed lung and bone or by esophageal access route. As a result, operators are forced to use X-ray fluoroscopy to guide catheter manipulation in contemporary repair of complex atrial and ventricular septal defects, valve leaflets, valve replacement, paravalvular leak, and left atrial appendage closure; operators must struggle visually to integrate 2D images into a mental image of anatomic context during key steps of protracted and occasionally unsuccessful procedures. Moreover, current 3D TEE probes, although shown to be useful in repair of septal defects, are not small enough for young children. Miniaturization of ultrasound probes to provide uninterrupted real-time full-volume intraprocedural three-dimensional en face depiction of cardiac pathology and catheter devices would represent a dramatic advance in image-guided intervention